Although not as famous as nearby Grotto or Grand Geysers, Fan and Mortar Geysers are an eminent delight to visitors.
With around a dozen vents at its disposal, Fan and Mortar Geysers has been hailed as “truly spectacular,” with jets from Mortar potentially spanning 60 feet or more.
Nestled in the Upper Geyser Basin near Morning Glory Pool, this pair of geysers has thrilled visitors to Yellowstone for over a century. Indeed, according to Lee H. Whittlesey, writing in Yellowstone Place Names, the feature was named by members of the 1870 Washburn Expedition. Co-leader Nathaniel Langford included Fan in his description of the Upper Geyser Basin included in this diary of the expedition:
The “Fan” has a distorted pipe from which are projected two radiating sheets of water to the height of sixty feet, resembling a feather fan. Forty feet from this geyser is a vent connected with it, two feet in diameter, which, during the eruption, expels with loud reports dense volumes of vapor to the height of fifty feet.
Another explorer who fell under Fan’s charms included W.H. Holmes of the Hayden Geological Survey. Here’s Holmes account of Fan Geyser, published in Whittlesey:
On the opposite side of the river, which here is comparatively quiet and some 40 feet wide, stand two low geyser cones or piles set into the gray dome’s bank and projecting slightly into the river. The upper one suddenly ceased as I reached the bank and the lower one began to sputter; very quickly a splendid fan-shaped jet was thrown into the air… its thousand darted jets trembling from left to right. To the left, and beyond this, within 6 feet a second stream of steam water of unexampled beauty was projected into the air to the distance of 100 to 200 feet and what was most surprising was that it stood at an angle of 40 degrees, another little jet nearer me shooting but at a similar angle towards me. It is quite impossible to describe the wonderful spectacular display that followed or to give any adequate idea of the beauty of the two principal jets or of the resistless force with which they were projected upward.
According to the Geyser Observation and Study Association, Fan and Mortar Geysers were known to play separately during the Park’s early recorded history, although today they generally erupt together. From the GOSA:
Fan and Mortar Geysers’ intervals are quite erratic from month to month but in the short term they are often stable enough so that people are willing to sit and wait for an eruption. Intervals range from about 3 days to weeks. The geyser is known to go dormant for months at a time. Some years pass without any eruptions. Fan and Mortar often slow down or stop in the Spring and early summer leading some to theorize that high river levels may cause water to find its way into the geyser, which abuts the Firehole River. It is thought that this influx of water, assuming that there is such and influx, may quench the geysers and keep them from erupting.
Durations of an eruption last about one hour. During this time, the geysers will pause and restart a number of times. The initial part of the eruption, about the first ten minutes, is usually the strongest. Heights for Fan Geyser can reach 125 feet with a horizontal throw of over 200 feet. Mortar Geyser can reach over 80 feet.
Although Fan and Mortar Geysers are quite impressive, their eruption patterns, as mentioned, are historically erratic. From Whittlesey:
Both Fan and Mortar geysers went dormant in about 1916 and did not erupt much until 1929. From 1929 to 1968, there were occasional eruptions. In 1969, the geysers rejuvenated in grand style, erupting at least 79 times in two years. Since that time, Fan and Mortar have been sporadically active. To see them erupt is to see one of the most spectacular sights in Yellowstone.